Project summary: To understand learning and memory, we must first elucidate the molecular machinery that regulates synaptic plasticity in the CNS. Recent results indicate that long-term potentiation (LTP), a major form of synaptic plasticity, is not a unitary process, even at a single synapse. LTP is expressed by independent presynaptic and postsynaptic mechanisms at excitatory synapses between CA3 and CA1 pyramidal neurons in the hippocampus. Given that the induction of plasticity at these synapses involves the influx of calcium ions into postsynaptic CA1 neurons, the expression of the presynaptic component of LTP suggests the existence of a retrograde signal that transfers a signal from the postsynaptic CA1 neuron to the presynaptic CA3 cell. Various molecular mechanisms have been implicated in retrograde signaling during LTP; however, none alone satisfies all of the requirements. In this proposal, we will examine the roles of nitric oxide, integrins, and neuronal adhesion molecule L1 in mechanisms of retrograde signaling during LTP. To accomplish these goals, we will directly visualize functional changes using fluorescent indicators of presynaptic function at the level of a single synapse. Two-photon laser scanning microscopy and 2-photon laser scanning uncaging techniques will be used in combination with electrophysiological tools in acute hippocampal slices from transgenic mice expressing various fluorescent markers in pyramidal neurons. Relevance: Long-term potentiation (LTP) is the use-dependent enhancement of the signal between neurons in the brain. LTP is thought to be a key process that regulates learning and memory. To better understand learning and memory and further the discovery of ways to prevent disease from disrupting these essential abilities in humans, we will investigate the molecular bases of LTP in mice with appropriate genetic mutations.